1. Field of the Invention
The invention relates to memory systems and memory access methods, and more particularly to memory systems and memory access methods using on-die terminations.
2. Description of the Related Art
Conventional computer systems generally include a processing unit, such as a microprocessor, dynamic memory units, data storage units, and other peripheral units, such as I/O. A circuit board is provided for interconnecting these various functional units. Some of the functional units are installed directly on the circuit board, while others are connected to the circuit board through expansion sockets. For instance, a particular circuit board may have surface-mounted memory modules, memory modules that interact with one or more expansion sockets, or a combination of both.
Commonly used memory modules that interact with expansion sockets through edge connectors are multi-chip modules (MCMs). Generally, MCMs may be designed to include more than one type of die within a single package, or may include multiples of the same die, such as a single inline memory module (SIMM) or a dual in-line memory module (DIMM). MCMs are available in different capacities and speeds to accommodate the needs of the computer system.
The memory, whether installed directly on the circuit board or through expansion sockets, are coupled to a common transmission bus, where data, address, and command signals are provided in parallel to the various modules. The operating environment (e.g., speed, bus load) and physical characteristics (e.g., length) of the transmission bus affect the characteristics of the transmission bus. In addition, electrical signals are reflected back when they reach the end of a transmission path. Electrical signals can also be reflected at points where impedance differs, such as at bus and DRAM connection points.
One technique for reducing the effects of reflections on the transmission bus is to install a termination circuitry connected to the transmission bus on the circuit board. However, a relatively large surface area of the circuit board is required by the termination circuitry, increasing the cost of the circuit board and reducing available area for other resources. In addition, more vias are required for installing the termination circuitry, increasing layout complexity and the delay of signals.